Conventionally, a liquid crystal display apparatus has had a problem of low response speed. That is, a change in display gradation of the liquid crystal display apparatus is such that: a change in orientation state of liquid crystal molecules is made by making a change in voltage applied to a liquid crystal layer, so that the transmittance of a display pixel is changed. Moreover, the low response speed of the liquid crystal display apparatus is attributed to the fact that it takes a long time to complete the change caused in orientation state of the liquid crystal molecules in response to the change in voltage applied to the liquid crystal layer.
In recent years, liquid crystal display apparatuses such as liquid crystal televisions, portable televisions, and portable game machines have had increased opportunities to display high-definition moving images with liquid crystals, and therefore have been increasingly required to respond at high speeds. On the other hand, high-quality picture technologies often cause a decrease in response speed simultaneously (e.g., AVS and mobile AVS).
As disclosed in Japanese Unexamined Patent Application Publication No. 78129/2004 (Tokukai 2004-78129; published on Mar. 11, 2004), a known example of a method for attempting to improve response speed is a method for emphasizing a transitional gradation by performing overdrive driving. That is, as shown in FIG. 12, the overdrive driving is such that when the initial luminance A of the initial gradation 0 is changed to the target luminance C of the target gradation 64, a voltage corresponding to the excessive luminance B, which is higher than the target luminance C, is initially applied to the liquid crystals only for a short time. This causes a high voltage to be applied to the liquid crystals, thereby making it possible to reduce the response time it takes to attain the target luminance C.
However, as shown in FIG. 9, such a method causes deterioration in image quality. Examples of such deterioration in image quality include a so-called angular response (two-step response) which, before the target luminance C is attained, emerges as a sharp corner indicating the excessive luminance B, which is higher than the target luminance C. The presence of such a corner indicating a luminance higher than the target luminance C causes an image to instantaneously look whitish. Since this is very conspicuously identified, it is necessary that the driving be performed so that no such corner emerges.
However, a change in overdrive amount only causes a change in size of the left angular portion, and does not result in an improvement in the right sloping portion. Therefore, there is no improvement in display. Further, as described above, an excessive overdrive amount causes a strikingly white display to be produced at the angular portion, thereby causing deterioration in display quality.
Furthermore, even when the overdrive driving is performed, a sufficient speed may not be able to be obtained in a low-gradation region due to the aforementioned low response speed.
That is, the aforementioned low response speed of a liquid crystal display apparatus is not seen uniformly all over the gradation-level regions, but is such that the response speed becomes extremely low in part of the gradation regions. For example, the response speed of a vertically-aligned and normally black liquid crystal display apparatus (mobile ASV) is extremely low at a rising edge from a low gradation (black display) to an intermediate gradation. Further, the response speed of a normally white liquid crystal display apparatus (mobile ASV) is extremely low in a transition from a high gradation (white display) to an intermediate gradation. These low response speeds cause display problems such as residual images.
In view of this, for example, Japanese Unexamined Patent Application Publication No. 131721/2002 (Tokukai 2002-131721; published on May 9, 2002) discloses a method for improving response speed by carrying out a display without using a gradation level at which the response speed becomes low. Specifically, the liquid crystal driving method of Patent Document 2 tries not to use a gradation level, ranging from a high gradation (white display) to an intermediate gradation, at which the response speed of a normally white system becomes low. Note that a voltage so applied to liquid crystals to be used for driving a liquid crystal display apparatus is usually represented by a gradation-luminance curve shown in FIG. 13.
However, according to the conventional method of Tokukai 2002-131721 for driving a liquid crystal display apparatus, the initial voltage is increased by a predetermined voltage when a gradation level at which the response speed becomes low is not used. Therefore, when a still image is displayed, it is impossible to use a normal luminance characteristic represented by the gradation-luminance curve of FIG. 13.
The present invention has been made in view of the foregoing problems, and it is an object of the present invention to provide a method for driving a liquid crystal display apparatus, which method makes it possible to prevent deterioration in display quality of both a still image and a moving image and to achieve an improvement in response speed at which a moving image is displayed.